1. Field of the Invention
The invention relates to nanoparticulate zinc oxide, coated with a phosphonocarboxylic acid, and to the use of nanoparticulate zinc oxide.
2. Discussion of Background Information
The importance of water-based binder systems such as coating systems in industrial application is becoming increasingly greater. Reasons for this are primarily statutory impositions relating to the environment-friendliness of the coating systems and binder systems to be used, an example being the VOC Directive. However, water-based systems still often show disadvantages over solvent-based systems, such as poor blocking resistance or long drying times, for example, which severely delay the further-processing or utilization of, for example, painted parts.
Zinc oxide is already used for various purposes in water-based paints. Its use as a white pigment is well known. In wood paints, ZnO, alongside other zinc salts, is used as an additive in order to prevent the bleeding of tannic acid. It is also used in primers on metals, as an adhesion promoter. These applications utilize zinc oxide particles having a diameter of more than 1 μm.
Zinc oxide and other metal oxides are also used in paints in order to enhance UV protection, solvent resistance, and surface hardness. WO 2008/049679 describes the use of nanoparticles for paints on an organic solvent basis in order to achieve an improvement in the scratch resistance and in the flexibility. U.S. Pat. No. 6,342,556 describes the use of ZnO particles having a diameter of 10 to 80 nm in water in combination with paints for enhanced UV protection of wood.
WO 2005/071002 relates to the use of a dispersion comprising ZnO having a BET surface area of 10 to 200 m2/g as transparent UV protection in paints. The ZnO particles described in this application are pyrogenically prepared particles which in dispersion have an average secondary particle size of less than 300 nm. These pyrogenically produced particles do possess a specific surface area in the order of magnitude of nanoparticulate zinc oxide, but cannot be used to produce transparent coats. The additive specified in WO 2005/071002, therefore, is unsuitable for use in transparent coating systems.
WO 2005/071029 uses zinc oxide having a particle diameter of less than 100 nm in order to enhance the surface hardness and the scratch resistance of polyurethane (PUR) paints. WO 2006/023064 describes the use of ZnO dispersions in PUR binders for producing scratch-resistant floor coatings. WO2006/023064 describes the production of scratch-resistant wood preservative paints through the use of ZnO dispersions in acrylic binders.
WO 2006/023064 describes the use of 2% to 20% of nanoscale zinc oxide in acrylate-containing paints and coatings in order to improve, for example, the adhesion, tannin resistance or corrosion resistance. Since the fraction of the relatively costly ZnO is more than 2%, the composition is often difficult to establish economically for the topcoat segment. WO 2006/023065 relates to coating compositions which comprise microscale TiO2 pigments and nanoscale metal oxide such as ZnO, the purpose of the nanoscale metal oxide being to ensure uniform spacing of the TiO2 pigments in the coat.
WO 2005/119359 and US-A1-2003/0180466 describe film-forming compositions which comprise binder, nanoparticle, surfactant, and a polymeric dispersant, in order to improve the abrasion resistance of substrates coated with said compositions.
WO 2008/064114 relates to the use of ZnO particles having a diameter of less than 1 μm in an acrylate hybrid paint for improving the solvent resistance, the moisture resistance, and the UV resistance.
DE 102007032189 A1 describes a specific nanoscale ZnO, prepared by sol-gel methods in organic solvents, it being possible for the ZnO to be surface-modified with Si-containing molecules. Resultant ZnO particle dispersions in organic solvents are utilized as a catalyst in order to increase the crosslinking rate in—for example—two-component PU paints. A disadvantage, in addition to the costly and inconvenient production method, is that the system can be employed only for solventborne paints and only for two-component systems. The severely curtailed potlife is a further disadvantage, since the time window for further-processing is significantly restricted.
For producing stable coating dispersions and binder dispersions which comprise ZnO it is necessary for the ZnO particles to have a like charge. Water-based binder systems, including coating compositions and latex dispersions, are frequently composed of anionically stabilized particles. Many of these dispersions are formulated at pH levels of between 7 and 9. The isoelectric point of ZnO is situated at a pH of around 9-9.5. Therefore, ZnO particles with a pH of less than 9 are present in positive and hence cationic form. Formulating the pH of a ZnO dispersion above the isoelectric point, in order thus to obtain negatively charged particles, is not possible, owing to the amphoteric nature of the zinc oxide, which then dissolves.
DE 10163256 A1 discloses a zinc oxide surface-modified with an oligo- or polyethylene glycol acid. The zinc oxide surface-modified in this way can be dispersed in water. A disadvantage is the modification of the zinc oxide in a nonaqueous environment. In order to obtain an aqueous dispersion, the organic solvent must first be removed and then the ZnO modified with an oligo polyethylene glycol acid must be dispersed in water again. The application of the dispersion is restricted on account of the positive zeta potential.
One possible way of stabilizing ZnO is to shift the isoelectric point toward lower pH levels. This can be done using polyelectrolytes. WO A 95/24359, for example, uses the sodium salt of a polyacrylic acid as a milling additive in the milling of zinc oxide. E. Tang et. al., in Appl. Surf. Sci., 252, 2006, 5227-5232, describe the use of a polymethacrylic acid having a molar mass of less than 10 000 g/mol in water, which is adjusted with NaOH to a pH of 5.0-5.5, for stabilizing an aqueous zinc oxide dispersion. For the preparation of stable nanoscale dispersions, large amounts of the polymethacrylic acid are required, such as 10% to 50%, based on the nanoscale powder. In addition there is a tendency toward bridging at high solids fractions.
WO 2007/082155 relates to the use of salts of 2,3-dihydroxynaphthalene-6-sulfonic acid, and catechol-3,5-disulfonates, for dispersing zinc oxide in the presence of ethylene glycol, propylene glycol, glycerol or the monoethers thereof. This is another means for shifting the isoelectric point toward lower pH levels. A disadvantage is that the dispersions undergo severe discoloration, particularly under the influence of light or in the presence of transition metal ions, especially iron ions, and this severely restricts the application of the ZnO dispersions in paints.
Another possible way of shifting the isoelectric point toward lower pH levels is to envelop the particles with a layer of silica. SiO2 has an isoelectric point of 2 and is negatively charged above this pH level. DE 10118309 C2 describes the preparation of an anionically stabilized, aqueous dispersion of nanoparticulate zinc oxide and the use thereof as a vulcanization activator for the vulcanization of latex moldings. The negative charge on the particle there is achieved by envelopment with silicates. For this purpose, at pH levels below the isoelectric point, zinc oxide is dispersed in water and alkali metal silicate solutions or waterglass/base mixtures are added, and so ZnO is converted to an anionic charge. A disadvantage is the growth rate of the silicates, which must be controlled such that no relatively large agglomerates are formed. Ions introduced by the alkali metal silicate must subsequently be removed from the dispersion, in order to obtain a sufficient shelflife at economical concentrations.
Phosphonates are used industrially in cooling-water systems, in desalination plants, and in oil extraction, where they prevent the precipitation of salts. In the paper and textiles industries, they are used as a stabilizer for peroxide bleaching, thereby complexing metals which otherwise deactivate the peroxide. 2-Phosphono-1,2,4-butanetricarboxylic acid is used in the treatment of cooling water and process water, and also in the sector of cleaning formulations, as an inhibitor both of scale and of corrosion.
EP 0760387 B1 relates to an anticorrosive pigment comprising a metal organophosphonate. The pigments are prepared by reacting organophosphonic acids or phosphonocarboxylic acids with suitable polyvalent metal cations, in the form, for example, of oxides, hydroxides, carbonates, chlorides, nitrates or sulfates. When the oxides or hydroxides are used, a simple acid-based reaction takes place, the oxides or hydroxides being thereby converted into the corresponding phosphonate salts.
There is therefore a need for additives for aqueous binder systems, such as coating systems, for example, with which the drying time can be significantly shortened and the blocking resistance increased, without impairing the overall profile of the binder system through the addition of large quantities. Even small quantities of less than 2% by weight are to achieve a significant effect here, in order not adversely to affect the production costs, and in order to detract as little as possible from the other properties of the binder system, especially of a paint.
The object of the invention, therefore, was to provide a material which enhances the drying properties and the blocking behavior of water-based binder systems, more particularly coating systems.